Heat shrinkable film structures with improved sealability and toughness

ABSTRACT

A multi-layer packaging film comprising layers having varying degrees of cross-linking when subjected to electron beam (EB) radiation. The outer layer has a high degree of cross-linking to improve the adhesion, strength, toughness and heat resistance of the film and the inner layer has a low degree of cross-linking to improve the sealability. Bags made from the multiple layer films are especially useful for shrink packaging, and particularly for shrink packaging of meats having large cavities.

This application is a continuation of application Ser. No. 08/884,121,filed Jun. 27, 1997, now U.S. Pat. No. 6,051,292 which was continuationof application Ser. No. 08/487,868, filed Jun. 7, 1995, now abandonedwhich was a continuation of application Ser. No. 08/011,528, filed Jan.29, 1993, now abandoned.

BACKGROUND OF THE INVENTION

Heat shrinkable polymer films have gained substantial acceptance forsuch uses as the packaging of meats and other articles of food. Thisdescription will detail the usage of films for packaging meat; it beingunderstood that these films are also suitable for packaging otherproducts such as frozen foods and cheeses. Some of the films embodyingthis invention are normally used as heat shrinkable bags supplied to themeat packer with one open end, to be closed and sealed after insertionof the meat. After the product is inserted, air is normally evacuated,the open end of the bag is closed, such as by heat sealing, or applyinga metal clip, and finally heat is applied, such as by hot water, toinitiate film shrinkage about the meat.

In subsequent processing of the meat, the bag may be opened and the meatremoved for further cutting of the meat into user cuts, for retail sale,for example, or for institutional use.

Successful shrink bags must satisfy a multiplicity of requirementsimposed by both the bag producer and the bag user. Of primary importanceto the bag user is the capability of the bag to survive physicallyintact the process of being filled, evacuated, sealed closed, and heatshrunk. The bag must also be strong enough to survive the materialhandling involved in moving the contained meat, which may weigh 100pounds or more, along the distribution system to the next processor, orto the user. Frequently, the meat will have protruding bones that willpuncture shrink bags that are not sufficiently strong. Thus, the bagmust be strong enough to physically protect the meat.

It is also highly desirable to the bag user that the bag serve as abarrier to infusion of gaseous material from the surroundingenvironment. Of particular importance is provision of an effectivebarrier to infusion of oxygen, since oxygen is well known to causespoilage of meat.

The bag producer requires a product which can be produced competitivelywhile meeting the performance requirements of the user. Thus, the bagmaterial should be readily extrudable, and susceptible to cross-linkingby irradiation, with sufficient leeway in process parameters as to allowfor efficient film production. The process should also be susceptible toefficient extended production operations. In the irradiation process,the film produced must have improved strength while the inner layermaintains sufficient sealability.

It is important to users of shrink bags that once the meat or other foodarticle is placed in the bag, the bag can be heat sealed to form an airtight bond. Generally, the heat seal is formed by applying heat andpressure to the opposing sides of the bag mouth until the opposing innerlayers of the film have fused together.

One of the problems encountered when heat sealing bags made frommultiple layer films is that the sealing process causes the film tobecome deformed in the area where the heat is applied. A solution tothis problem known in the art has been to cross-link the film layers byirradiation prior to heat sealing. Cross-linking the film providesimproved toughness and increases the heat seal temperature range.

However, cross-linked thermoplastic films are more difficult to melt andproduce weaker seals than unirradiated films when heat sealed. Usersrequire that the seals maintain their integrity when the bag containingmeat or other food article is immersed in hot water to shrink the film.A bag with weak heat seals that rupture when the bag is shrunk, is of nouse. Thus, there is a need for an irradiated multiple layer film whichcan be made into a bag that will have strong seals when heat sealed.

It is known that heat shrinkable bags for food packaging may be madefrom multiple layer films in which individual layers have differentdegrees of cross-linking. Such multiple layer films have been fabricatedby forming and irradiating the layers individually and then laminatingor extrusion coating the layers to form the multiple layer film. Thesemultiple step fabrication methods produce a more costly film.

Canadian Patent, 1,125,229 discloses a film structure having a heatsealable inner layer and an outer layer wherein the outer layer iscross-linked to a larger extent then the heat sealable layer. Thedifferential cross-linking is achieved by adding a cross-linkingenhancer compound to the outer layer, forming the structure, and thenirradiating. The irradiation enhancer allows the irradiation dosage tobe lowered to a point where the heat sealable inner layer is notadversely affected in its heat sealing characteristics by the radiation.However, the lower irradiation dosage does not produce a bag with thestrength and toughness required by users.

U.S. Pat. No. 4,724,176 to Sun, discloses a heat shrinkable containerhaving an unirradiated inner layer, a barrier layer and an irradiatedouter layer. This film is coextruded and the outer layer is irradiatedby precisely controlling the depth of material that the electron beamirradiation penetrates. This invention limits the cross-linking to theouter layer and therefore, does not improve the strength of the innerlayer by cross-linking.

U.S. Pat. No. 5,055,328 to Evert discloses a multiple layer film inwhich the inner layer contains an antioxidant cross-linking inhibitor tocontrol the degree of cross-linking.

U.S. Pat. No. 4,894,107 to Tse discloses oriented or unoriented multiplelayer films with barrier layers that are irradiated for cross-linking.This invention does not teach the selection of polymers for individuallayers with different degrees of cross-linking when irradiated.

Many of the multilayer heat shrinkable films with an uncross-linkedinner layer and a cross-linked outer layer previously known in the artwere made by extruding the outer layer separately from the inner layer.After the outer layer was extruded and irradiated, it would be laminatedwith the inner layer and any other additional layers to form themultilayer film. In the present invention, the additional expense ofseparately extruding the irradiated and unirradiated layers and thenlaminating the layers together is avoided. All of the layers of thefilms in the present invention can be coextruded and the entire filmstructure can be exposed to EB radiation. Such films would be widelyaccepted by those skilled in the art and meet with substantialcommercial success.

It is an object of this invention to provide multiple layer cross-linkedfilms with improved sealability and toughness. It is a further object ofthis invention to provide a coextruded multiple layer cross-linked filmhaving these improvements after subjecting the fabricated multiple layerfilm structure to a single dose of irradiation. Another object of thisinvention is to provide a meat or food article packaging bag that willmaintain the integrity of heat seals when it is shrunk wrapped.

It should be understood that the objectives stated in or inferred fromthis specification do not limit the invention as defined in the claims.

Irradiation of polymers causes the formation of covalent bonds betweendifferent polymer chains. This process is called “cross-linking”. Theoverall effect of cross-linking is that the molecular weight of thepolymer steadily increases with dose, leading to branched chains, untilultimately a tri-dimensional network is formed. This three dimensionalnetwork is referred to as the “gel fraction”. The gel fractionsdisclosed herein have been determined in accordance with ASTM D2765. Thegel fraction molecules are insoluble while the unlinked molecules remainsoluble and are referred to as the “sol fraction”. These molecules areseparated from the network although they may be highly branched and canbe extracted from the bulk polymer by a process that uses a propersolvent. Thus, the gel fraction can be easily measured to determine theextent of cross-linking for various radiation doses.

It has been known in the art that irradiation of polymeric multiplelayer films cross-links the layers and produces a film with improvedtoughness and strength. However, cross-linking also raises the normalmelting temperature of a polymer and consequently reduces the heatsealability. Surprisingly, it has been found in the practice of thisinvention that by the selection of different polymers for the variouslayers of a multiple layer film, it is possible to have extensivecross-linking in one layer and a minimum amount of cross-linking inanother layer when the film is irradiated. This allows the outer layerof a multiple layer film to be cross-linked to provide increasedstrength and toughness while the inner layer is not cross-linked andretains its heat sealability characteristics.

SUMMARY OF THE INVENTION

The multiple layer films in the invention have inner heat sealant layersand outer protective layers that have different degrees of cross-linkingwhen subjected to electron beam irradiation. More significantly, thesefilms experience incipient cross-linking at different levels ofirradiation doses. Thus, by the selection of the materials for theprotective and heat sealant layers as taught by this invention, it ispossible to form an irradiated multiple layer film having a protectivelayer with significant cross-linking and a heat sealant layer with onlyminimal cross-linking.

The irradiation serves at least two significant purposes. First, itenhances the heat resistance of the protective layer of the film. Thisis evidenced by reduced failure rates in packages which have been heatshrunk or heat sealed. Second, the timing of the irradiation treatmentbeing after the formation of the multiple layer film, substantialfreedom is available in selecting the process for fabricating themultiple layer film. Thus the processes which tend to yield higherinterfacial adhesion, such as coextrusion, are preferred. Because moredesirable formation processes can be used, the resulting films may havesubstantially improved interfacial adhesion over similar films made byless desirable processes.

The amount of cross-linking in the protective and heat sealant levers ismeasured by the gel fraction of the material after irradiation. Thehigher the gel fraction, the greater the amount of cross-linking.Irradiation doses of from about 2 MR to about 10 MR are used tocross-link the films of this invention. The most preferred irradiationdose for the invention is from about 4 MR to about 6 MR. Within thisrange, the protective layer undergoes significant cross-linking thatmakes the film tougher and gives it added strength, while the heatsealant layer undergoes an insignificant amount of cross-linking andmaintains its heat sealability.

A substantial end use for the films of the invention is in thefabrication of heat sealable shrink bags that are particularly useful inthe packaging of meat, especially meat having bony projections or largecavities. Bags made according to this invention find particular utilityin forming packages which are subjected to high temperature shrinkingprocesses.

The bags produced from the films in this invention have the followingadvantages over bags known to the art: 1) the bags are tougher andexhibit superior puncture resistance; 2) the bags have a higher heatseal strength; 3) the bags have a higher burst value; and 4) theincreased toughness and higher heat seal strength allow the bag machinesto be operated at faster speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a three-layer film of the presentinvention.

FIG. 2 is a cross-sectional view of a five-layer film of the presentinvention.

FIG. 3 is a plan view of a bag made according to the invention.

FIG. 4 is a cross-sectional view of the bag of FIG. 3 taken at 2—2 ofFIG. 3.

FIG. 5 is a cross-sectional view of the bag of FIG. 3 taken at 2—2 ofFIG. 3, except the film is a 5 layer structure.

FIG. 6 is a graph of gel fraction versus radiation dose for EVA andULDPE.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, it has been found that good heatsealability can be obtained in a multi-layer film that has beenirradiated. In one embodiment of the invention, a material in the innerheat sealant layer, such as ultra low density polyethylene (ULDPE), lowdensity polyethylene (LDPE), linear low density polyethylene (LLDPE),very low density polyethylene (VLDPE), or ionomers such as Surlyn,having incipient cross-linking at a higher radiation dose level than thematerial in the outer protective layer, produces an irradiated film withsuperior heat sealability. In one embodiment of this invention, theouter protective layer is comprised of a blend of at least 50% of an EVAresin. The inner sealant layer may be comprised of a copolymer of atleast 50% ULDPE, LDPE, LLDPE, VLDPE or ionomers such as Surlyn. TheULDPE is commercially available from Dow Chemical Company under thetrade name Dow 4201 and the EVA resin is commercially available fromExxon as XV-97.06, LD-306.09, LD-318.92 or LD-701.06 and DuPont as ELVAX3129. Interposed between the inner and outer layers is a gas barrierlayer.

In preferred embodiments of this invention, the outer layer comprises atleast 70% EVA resin and in other embodiments the outer layer comprisesat least 80% and at least 90% EVA resin.

The multiple layer films of this invention are subjected to electronbeam (EB) radiation with preferred doses in the range of from about 2megarads to about 10 megarads (MR).

In another preferred embodiment of this invention, the films aresubjected to EB radiation doses of from about 2 MR to about 6 MR.

In a particularly preferred embodiment of this invention, the films aresubjected to EB radiation doses of from about 4 to about 6 MR.

The multiple layer films of this invention are comprised of at least twolayers. In a more preferred embodiment, the film is provided with anoxygen or flavor barrier core layer. The oxygen or flavor barrier layeris preferably a vinylidene chloride copolymer such asmethyl-acrylate-polyvinylidene chloride (MA-PVdC) or ethylene vinylalcohol (EVOH). However, it is not limited to these materials. There mayalso be present in the film an oxygen absorbing or oxygen scavengingmaterial. Examples of such materials are disclosed in U.S. Pat. Nos.4,536,409 and 4,702,966 to Farrell, U.S. Pat. No. 4,856,650 to Inoure,U.S. Pat. No. 4,919,984 to Maruhashi, U.S. Pat. No. 5,021,515 toCochran, and U.S. Pat. No. 4,877,664 to Maeda and U.S. patentapplication Ser. No. 07/761,490 by Kim, the disclosure of which areincorporated herein by reference. In addition to the oxygen or flavorbarrier layer and inner and outer layers, the film can also be comprisedof adhesive layers and additional layers to improve the strength andtoughness of the films.

The multiple layer films in the invention can be fabricated byconventional processes known in the art. These films can be eitheroriented or unoriented. A preferred process includes the steps ofcoextrusion of the layers to be oriented, followed by orientation in oneof the conventional processes such as blown tubular orientation orstretch orientation in the form of a continuous sheet; both beingmolecular orientation processes. Once the multiple layer film has beenformed and oriented, it is subjected to electron beam irradiation.

One of the advantages of the films of the present invention is that theycan be coextruded and there is no need to laminate layers. Similar filmsknown in the prior art were produced by extruding the outer layer andcross-linking it by irradiation before laminating it onto the innerlayer which had been extruded separately. Thus, the ability to coextrudethe inner and outer layers of the films of the present invention andexpose the entire film structure to EB radiation provides the user witha significant cost savings.

The amount of electron beam irradiation is adjusted, depending on themake-up of the specific film to be treated and the end use requirements.While virtually any amount of irradiation will induce somecross-linking, a minimum level of at least 1.5 megarads is usuallypreferred in order to achieve desired levels of enhancement of the hotstrength of the film and to expand the range of temperatures at whichsatisfactory heat seals may be formed. While treatment up to about 50megarads can be tolerated, there is usually no need to use more than 10megarads, so this is a preferred upper level of treatment; a preferreddosage being from about 2 megarads to about 6 megarads and the mostpreferred dosage being from about 4 megarads to about 6 megarads.

The film is subjected to electron beam irradiation only after themultiple layer film has been formed, and after molecular orientation, inthose embodiments where the film is molecularly oriented. It should benoted that, in the irradiation step, all of the layers in the film areexposed simultaneously to the irradiation source, such that irradiationof all the layers of the film takes place simultaneously.

FIG. 1 shows a three layer coextruded film made according to theinvention. Layer 14 is a barrier layer which minimizes the transmissionof oxygen through the film. The preferred barrier material is MA-PVdC,EVOH, nylon or a VdC-VC copolymer. Layer 16 is the heat sealant layerand it is composed of a polymer having a low degree of cross-linkingwhen subjected to irradiation. A preferred material for the heat sealantlayer is ULDPE sold by Dow Chemical Company as Dow 4201. However, othercopolymers, polymers and blends thereof with high dosage incipientcross-linking characteristics may be used (for example Exxon's Exactpolymer and Mitsui's TAFMER). Layer 18 is the outer protective layercomposed of a blend of at least from about 50% to about 100% EVA andfrom about 0% to about 50% ULDPE, VLDPE, LDPE, LLDPE or ionomers such asSurlyn. The preferred blend is from about 80% to about 90% EVA and fromabout 10% to about 20% ULDPE, VLDPE, LDPE, LLDPE or inomers such asSurlyn. The EVA in these blends can be comprised of one or moredifferent types of EVA. One preferred blend is comprised of Exxon'sLD-701.06 EVA or DuPont's ELVAX 3129 EVA, Dow Chemical's ATTANE 4201ULDPE and Exxon's LD-318.92.

FIG. 2 shows a five layer coextruded film made according to theinvention. Layer 114 is a barrier layer similar to layer 14. Layer 116is a hear sealant layer similar to layer 16. Layer 118 is an outerprotective layer similar to layer 18. Layer 120 is a firstprotective-adhesive layer composed of a blend of an EVA copolymer,preferably an EVA resin with 10% VA. Layer 122 is a secondprotective-adhesive layer comprising a polyethylene copolymer,preferably ULDPE, VLDPE, LDPE, LLDPE or ionomers such as Surlyn. Theadditional layers in the five layer film structure provide addedadhesion, strength, heat resistance and toughness.

Preferred EVA's are those having 6% to 12% vinyl acetate (VA) contentand having incipient cross-linking occur at about 1 to 2 MR and a gelfraction of at least 0.15 at 6 MR. Most preferred EVA's are EVA's havinga vinyl acetate content of about 10%.

Preferred materials for the heat sealant layer are those havingincipient cross-linking occur at about 5 MR or higher.

FIG. 3 shows a bag 10 made according to the invention. The empty bagshown is a collapsed, biaxially oriented tube with one end closed by aheat seal 12 across the one end of the tube. The other end of the bag isopen for insertion of meat or other food article, and it is normallyclosed and sealed when the meat or food article is put into the bag.

The cross-section of the bag in FIG. 4 shows a typical structure wherethe bag 10 is made from a three-layer coextruded plastic film. The heatseal 12 is formed by the application of heat and pressure to fuse theheat sealant layers 16 of the two multiple layer films that form theopposing walls of the bag 10.

The cross-section of the bag in FIG. 5 shows a typical structure wherethe bag 110 is made from a five-layer coextruded film. The heat seal 112is formed by the application of heat and pressure to fuse the heatsealant layers 116 of the two multiple layer films that form theopposing walls of the bag 110.

The overall thickness of films of this invention is nominally the sameas the thickness of conventional films. Films are generally about 2.0mils thick with a normal range of 1.0 to 4.0 mils. Films thinner than1.0 mils tend to be too weak to perform all required functions. Filmsthicker than 4.0 mils are economically unable to compete with thinnercompetitive films.

Table A shows the structure of typical 3-layer films of this invention.

TABLE A THICKNESS AND STRUCTURE OF TYPICAL THREE LAYER FILMS LAYERMATERIAL THICKNESS (× 10⁻² Mils) 1. Sealant ULDPE 110 Barrier MA-PVdC 30 Protective EVA/ULDPE  80 220 2. Sealant ULDPE 100 Barrier EVOH  30Protective EVA/ULDPE 100 230 3. Sealant ULDPE/EVA  80 Barrier MA-PVdC 30 Protective EVA/ULDPE/EVA  80 190 4. Sealant ULDPE/EVA  90 BarrierEVOH  30 Protective EVA/ULDPE/EVA  60 180

Table B shows the structure of typical 5-layer films of this invention.

TABLE B THICKNESS AND STRUCTURE OF TYPICAL FIVE LAYER FILMS LAYERMATERIAL THICKNESS (× 10⁻² Mils) 1. Sealant ULDPE  30 Second ProtectiveEVA 110 Barrier MA-PVdC  30 Adhesive ULDPE  30 First ProtectiveEVA/ULDPE  30 230 2. Sealant ULDPE  80 Second Protective EVA/ULDPE  30Barrier MA-PVdC  30 Adhesive ULDPE  30 First Protective EVA/ULDPE/EVA 60 230 3. Sealant ULDPE/EVA  60 Second Protective EVA/ULDPE/EVA  30Barrier EVOH  40 Adhesive ULDPE/EVA  30 First Protective EVA/ULDPE/EVA 60 220 4. Sealant ULDPE/EVA  80 Second Protective EVA/ULDPE  60 BarrierEVOH  20 Adhesive ULDPE/EVA  30 First Protective EVA/ULDPE/EVA  80 270

Table C shows the gel fractions for ULDPE and EVA at various irradiationdoses. These results are plotted on a graph as shown in FIG. 6.

TABLE C IRRADIATION GEL FRACTION EB, DOSE, MRAD ULDPE EVA 2 0.03 0.18 30.03 0.42 4 0.02 0.38 5 0.03 0.50 6 0.04 0.50 8 0.35 0.62 10  0.41 0.6615  0.56 0.75

It is to be understood that the films and the materials disclosed aboveas well as other films and materials which are apparent in view of thisspecification are not to be considered a limitation of the presentinvention, the scope of which is defined by the claims.

THE EXAMPLES

To determine the physical properties of various five layer filmstructures, twelve films were fabricated. These twelve films are listedin Table D and the materials of the individual layers and theirradiation dosage they were exposed to are shown for each. (Note films1B, 2B, 6B and 8B are not listed because good heat seals could not beformed for these structures.) Also, listed as sample number nine is athree layer film known in the prior art that was used as a control forcomparing the test results of the five layer films.

The twelve five layer films and the three layer control film were testedto determine tear strength, percent haze, gloss, puncture resistance,minimum sealing temperature, seal durability and impact strength. Filmsamples 4A, 4B, 7A and 7B represent film structures of the presentinvention with 7A and 7B being preferred structures.

TABLE D COMPOSITION OF SAMPLE MULTILAYER FILMS OUTER OUTER OXYGEN INNERINNER SAMPLE PROTECTIVE TIE BARRIER TIE SEALANT IRRADIATION NO. LAYERLAYER LAYER LAYER LAYER (DOSAGE) 1A 3651 3651 3649 3651 XV-97.06 Medium2A 3651 3651 3649 3651 306.09 Medium 3A 3651 3651 3649 3651 318.92Medium 3B 3651 3651 3649 3651 318.92 High 4A 3651 3651 3649 3651 4201Medium 4B 3651 3651 3649 3651 4201 High 5A 3651 4201 3649 XV-97.06306.09 Medium 5B 3651 4201 3649 XV-97.06 306.09 High 6A 3651 4201 3649XV-97.06 318.92 Medium 7A 3651 4201 3649 XV-97.06 4201 Medium 7B 36514201 3649 XV-97.06 4201 High 8A 3651 4201 3649 XV-97.06 XV-97.06 Medium9 3651 — 3649 — 3651 Medium

WHERE:

Medium irradiation dosage is between 4 and 5.5 megarads and highirradiation dosage is between 5.5 and 6.5 megarads.

3651—is an EVA1/ULDPE/EVA2 blend. EVA1 is an EVA copolymer such as ExxonLD-701.06, XV-97.06, 318.92 or DuPont ELVAX 3129; ULDPE is a ULDPEcopolymer such as Dow ATTANE 4201 and EVA2 is an EVA copolymer such asExxon LD-318.92, LD-701.06, XV-97.06 or DuPont ELVAX 3129.

4201—is an ethylene alpha-olefin copolymer manufactured by Dow ChemicalCompany and sold as ATTANE 4201.

XV-97.06—is an EVA copolymer manufactured by Exxon Chemical Co. with 10%vinyl acetate (VA) and a melt index of 0.3.

LD-306.09—is an EVA copolymer manufactured by Exxon Chemical Co. with5.8% VA and a melt index of 2.0.

LD-318.92—is an EVA copolymer manufactured by Exxon Chemical Co. with9.0% VA and a melt index of 2.2.

3649—is a Saran comprised of a mehtylacrylate-polyvinylidene chloridecopolymer.

ELVAX 3129—is an EVA copolymer manufactured by DuPont having 10% VA anda melt index of 0.35 g/10 min.

LD-701.06—is an EVA copolymer manufactured by Exxon having 10% VA and amelt index of 0.19 g/10 min.

Tear Strength Test

The comparative tear strengths of the films was measured using anElmendorf DigiTear Tester, Model No. 65-200, available fromThwing-Albert Instrument Company, Philadelphia, Penn.

The test consisted of; 1) preparing seven representative samples of eachfilm; 2) clamping a test specimen in the TAIR clamps of the tester; 3)slitting the film with a razor blade to initiate a slit; 4) releasingthe pendulum of the tester which exerts a force on the opposing sides ofthe slit; and 5) recording the digital readout values of the tester. Thedigital readout value is the amount of force required to tear the filmand it is measured in grams.

Table E below shows the results of the tear strength tests for sevenspecimens of each of the twelve five layer films and the three-layercontrol film. Each of the specimens was tested for tear strength in themachine direction (MD) and the cross-machine direction (CMD) and theresults were averaged. The results indicate that the film structures ofthe present invention (samples 4A, 4B, 7A and 7B) have above averagetear strengths, especially in the machine direction.

TABLE E TEAR STRENGTH TEST Tear strength is measured in grams. MD CMD MDCMD MD CMD SAMPLE NO: 1A 2A 3A 128  96 112 112 128 128  96 128 112  48112 128 112 144 112  48 112 112 112 112 112  96 128 144 128 128 112  80112 112 112 128  96 112 112  96 112 144  96 112  96 128 AVERAGE: 114 126107  87 114 121 SAMPLE NO: 3B 4A 4B 116 132 152  64 160 184 128 100 148140 176  72 120 104 156  64 160 172 112 128 180 132 148 120 120 100 184 76 172  76 120 104 156  96 152 152 124 124 172 120 176  56 AVERAGE: 120113 164  99 163 119 SAMPLE NO: 5A 5B 6A 120 544 104 480  96 272 104 368116 192 176 256 108 288 108 592  96 256 128 304 112 400 128 224 128 416120 448 112 208 124 192 108 432 112 368 124 528 124 416 112 336 AVERAGE:119 377 113 423 119 274 SAMPLE NO: 7A 7B 8A 128 176 132 236  96 480 148220 148 140 104 288 152 192 156 172 156 448 168 116 156 176 112 304 172108 160 104 100 432 144 228 180 124 104 320 148 180 184 132 160 352AVERAGE: 151 174 159 155 119 375 SAMPLE NO: 9  88 212 104 176 220 304128 256 240 256 140 208 220 208 AVERAGE: 163 231

Puncture Test

The puncture strengths of the twelve five layer films and the threelayer control film were measured using a compression tester manufacturedby Instron Corp. of Canton, Mass. The puncture test measures the amountof force needed to rupture a film. This test allows various films to berated as to their resistance to being punctured by the contents of apackage or by objects outside the package.

The test procedure consisted of the following: 1) preparing five 3″square representative film samples without delaminations; 2) driving ahemispherical-faced probe having a 6 mm diameter through a film samplebeing held in a circular clamp with the probe in contact with the innersealant layer of the film; 3) attaching a crosshead to the end of theprobe that is opposite to the end that is in contact with the film; 4)positioning the probe, crosshead and sample holder on the Instroncompression cell, 5) compressing the crosshead and the sample holderuntil the probe punctures the sample film, and 6) recording thecompression force (measured in pounds) required to puncture the samplefilm.

The puncture strengths of each of the five samples of the five-layerfilms and the three-layer control film are measured and an averagepuncture strength calculated for each film. The test results are listedbelow in Table F. The results indicate that the films of the invention(samples 4A, 4B, 7A and 7B) have excellent puncture strengths ascompared to the other films tested.

TABLE F Puncture resistance measured in pounds per square inch. SAMPLENO: 1A 2A 3A 3B 4A 4B 14.9 14.6 15.1 14.5 15.5 16.3 14.9 14.7 15.5 14.616.5 15.5 15.2 14.2 15.8 14.0 15.4 16.0 15.7 14.3 16.0 14.2 16.4 14.915.9 14.1 15.2 13.8 16.8 17.0 AVERAGE: 15.3 14.4 15.5 14.2 16.1 15.9SAMPLE NO: 5A 5B 6A 7A 7B 8A 15.4 16.3 15.8 18.3 18.5 16.3 15.6 15.315.5 18.5 19.0 15.7 15.9 15.8 15.5 18.8 17.9 16.3 15.8 16.5 15.1 16.818.3 16.0 15.2 15.0 15.6 17.7 19.1 16.5 AVERAGE: 15.6 15.8 15.5 18.018.6 16.2 SAMPLE NO: 9 16.3 14.8 15.1 15.7 16.1 AVERAGE: 15.6

Minimum Seal Temperature Test

The minimum seal temperature (MST) is the lowest temperature at which aweld seal can be achieved for making packaging seals. This test measuresthe effect of the cross-linking caused by irradiation on the heat sealtemperature. Irradiation is known to elevate the heat seal temperatureand make it more difficult to from a seal. The MST for the twelve fivelayer films and the three layer control film were measured in accordancewith ASTM F-88 using a Sentinel Sealer.

The test was performed as follows: 1) two one inch widths of the samplefilm were cut in either the machine direction or the cross-machinedirection into strips; 2) the two strips were placed one on top of theother with the sealant layers of the opposing films face to face; 3) thetwo strips of film were positioned in the Sentinel Sealer and a pressureof 50 psig for a dwell time of 1 second at an initial seal temperatureof 200° F.; 4) the seal was inspected to determine if an adequate sealhad been formed; 5) if an adequate seal was not achieved, steps 1through 4 were repeated at a seal temperature 5° F. higher than theprevious test until an adequate seal was achieved; 6) the test wasrepeated at least four more times at the seal temperature that was foundto be adequate to confirm the result; and 6) the minimum sealtemperature was recorded for the sample film.

The results for the films tested are listed below in Table G.

TABLE G Minimum Seal Temperature Test (MST) Minimum sealing temperatureis measured in ° F. SAMPLE NO. MST 1A 250 2A 370 (film degradationobserved) 3A 245 3B 245 4A 270 4B 280 5A 260 5B 280 6A 250 7A 270 7B 2708A 270 9 245

Seal Durability Test

The seal durability test is used to determine the level of cross-linkingin a thermoplastic film resulting from electron beam (EB) radiation.This information is helpful in determining the suitability of a film foruse with particular seal equipment.

The seal durability test used the Vertrod Impulse Heat Sealer with amotorized strain gauge and consisted of the following: 1) preparing sixsamples of each of the twelve five-layer films and the three-layercontrol film by cutting strips 8 inches in length in the machinedirection and 1 inch wide in the cross-machine direction; 2) folding thesample strip in half along the cross-machine axis so that the sealantlayers are on the inside; 3) clamping a strip of sample film in placebetween the sealing jaws of the Vertrod Heat Sealer and attaching oneend to the strain gauge; 4) heat sealing the sample film by actuatingthe heater element and applying 30 psig pressure for a dwell time of 0.7seconds; 5) activating the strain gauge motor as the seal bar raises; 6)measuring the seal durability as the motorized strain gauge stretchesthe heat seal and determining the peak force required to stretch thefilm at the heat seal; 7) visually examining the seal for burn throughand/or stretching; and 8) recording the maximum value measured by thestrain gauge.

The results of the seal durability test are listed below in Table H. Theresults indicate that the films of the invention (samples 4A, 4B, 7A and7B) have average seal durability but are less likely to peel than mostof the other films tested.

TABLE H Seal Durability Test Seal Durability is measured in ounces.SAMPLE NO: 1A 2A 3A 3B 4A 4B 16.5 NB 12.5 NB 15.0 NB 15.0 NB 15.0 NB17.0 NB 15.0 NB 12.0 NB 13.0 NB 15.0 NB 13.5 NB 16.0 NB  9.5 P 10.5 NB12.0 NB 11.0 NB 13.0 NB 15.0 NB 14.0 P 13.0 NB 13.5 NB 15.0 NB 15.5 NB16.0 NB 13.0 P 10.5 NB 14.0 NB 14.0 NB 14.0 NB 14.5 NB 11.5 P 11.0 NB12.5 NB 15.5 NB 14.0 NB 14.5 NB AVERAGE: 13.2 11.6 13.3 13.3 14.2 15.5SAMPLE NO: 5A 5B 6A 7A 7B 8A 15.0 P 15.0 P 12.0 P 11.0 NB 13.0 NB 13.5 P15.0 NB 11.0 P 14.5 P 13.0 NB 11.5 NB 15.0 NB 12.0 P 16.0 P 16.5 NB 12.5NB 13.5 NB 14.0 P 10.5 P 17.5 NB 15.5 P 14.0 NB 13.0 NB 12.0 NB 10.0 NB15.0 P 16.0 P 10.0 NB 15.0 NB 25.0 P 16.0 NB 17.0 NB 12.0 P 10.0 NB 10.5NB 22.0 P AVERAGE: 13.1 15.2 14.4 11.7 12.7 16.9 SAMPLE NO: 9 12.0 NB12.0 NB 10.0 NB 11.5 NB 11.0 NB  7.0 NB AVERAGE: 10.6 NB = No breakafter peak force. P = Peeling of the layers was observed.

Haze Test

Haze is a basic measure of film clarity. Haze is defined as the amountof light that is scattered as light passes through a film.

The haze test was performed in accordance with ASTM D-1003 using aPacific Scientific XL-211 Hazegard System. The test was performed on thetwelve five layer sample films and the three-layer control film andconsisted of: 1) preparing samples for each film by cutting the filmsinto five 4 inch square specimens; 2) positioning a sample on theHazegard System; 3) measuring the percent haze using an integratingsphere to collect light scattered by the sample film; and 4) recordingthe percent haze.

The results of the haze test are recorded below in Table I.

TABLE I Haze Test Haze is measured as the percentage of scattered light.SAMPLE NO: 1A 2A 3A 3B 4A 4B 14.2 13.1 14.4 13.3 10.0 9.66 13.7 12.914.3 12.4 10.7 10.5 14.7 11.7 14.8 13.8 11.0 12.1 11.9 11.1 11.7 13.810.9 12.4 12.5 13.8 13.0 14.3 12.2 10.7 12.0 13.3 12.2 13.0 9.24 10.1AVERAGE: 13.2 12.6 13.4 13.4 10.7 10.9 SAMPLE NO: 5A 5B 6A 7A 7B 8A 10.610.9 11.2 9.37 13.5 9.35 13.2 10.9 11.5 10.3 11.6 11.2 12.5 11.8 12.611.5 12.7 11.2 12.0 13.6 10.3 11.8 10.5 11.7 12.4 11.8 10.5 10.8 9.8711.5 12.1 13.7 10.2 11.2 12.3 13.0 AVERAGE: 12.1 12.1 11.0 10.8 11.711.3 SAMPLE NO: 9 8.98 11.10 9.81 8.34 8.52 8.71 AVERAGE: 9.2

Gloss Test

Gloss is a surface optical property of a film relating to the deflectionof light. Gloss is measured as the percentage of light secularlyreflected from the surface of the film. The Macbeth Lab-Gloss instrumentwas used to measure the gloss of the twelve five-layer sample films andthe three-layer control film.

The gloss test consisted of the following: 1) preparing samples for eachof the films by cutting the films into 4 inch square specimens; 2)positioning a sample on the Macbeth Lab-Gloss instrument; 3) reflectinga beam of light from a light Source onto the surface of the film sampleat a 45 degree angle; 4) measuring the percentage of light reflectingfrom the surface; and 5) recording the results.

The gloss test results were recorded in Table J below.

TABLE J Gloss Test Gloss is measured as the percentage of lightreflected when a light strikes the surface at a 45 degree angle. SAMPLENO: 1A 2A 3A 3B 4A 4B 65.3 62.5 62.6 61.5 68.5 69.0 65.4 63.9 64.5 63.870.4 68.3 63.8 65.6 60.4 65.0 67.7 66.8 61.9 67.1 65.9 64.1 67.7 63.765.9 67.4 64.8 63.4 67.4 69.3 66.8 65.8 66.5 62.8 69.4 68.0 AVERAGE:64.8 65.3 64.1 63.4 68.5 67.5 SAMPLE NO: 5A 5B 6A 7A 7B 8A 69.3 67.468.3 64.8 61.0 67.9 69.9 69.3 65.0 68.6 63.8 68.9 65.2 66.2 69.1 64.763.2 68.1 64.3 68.8 68.3 62.8 60.9 68.3 64.0 69.3 70.0 67.2 64.4 68.668.0 68.1 68.7 67.5 61.5 69.5 AVERAGE: 66.7 68.1 68.2 65.9 62.4 68.5SAMPLE NO.: 9 68.2 68.9 67.2 68.5 63.9 68.7 AVERAGE: 67.5

Impact Strength Test

The impact strength test is used to determine the total energy impactstrength of a film structure by measuring the kinetic energy lost by afree-falling dart that passes through the film. This test is useful inpredicting the performance of a film used for packaging. The testsimulates the action encountered in applications where moderate velocityblunt impacts occur in relatively small areas of the film.

The impact strength test uses a Kayeness Energy Absorption Impact Tester(EAIT) and consists of the following: 1) preparing samples for each ofthe twelve five-layer films and the three-layer control film by cuttingthe films into five 8-inch square specimens; 2) positing a sample filmson the EAIT; 3) dropping a 35 pound probe having a 1½ inch diameter tipthrough the sample film; 4) measuring the force of the probe as itstrikes a load cell positioned below the sample film; 5) determining theimpact strength of the sample film by calculating the kinetic energythat is lost when the probe passes through the film; and 6) recordingthe test results.

The test results are listed in Table K below. The results indicate thatthe films of the invention (samples 4A, 4B, 7A and 7B) have aboveaverage impact strengths.

TABLE K Impact Strength Test The impact energy is measured infoot-pounds. SAMPLE NO. 1A 2A 3A 3B 5.96 5.25 6.05 4.83 5.49 5.95 6.335.76 5.45 5.84 6.33 4.79 5.47 6.07 4.92 5.56 6.07 6.00 5.83 AVERAGE:5.64 5.61 6.17 5.35 SAMPLE NO. 4A 4B 5A 5B 8.42 9.45 6.10 6.83 9.44 9.416.52 6.89 6.20 7.81 7.66 6.42 7.90 8.51 7.31 7.17 8.30 8.30 6.48 6.299.14 6.98 AVERAGE: 8.05 8.77 6.84 6.72 SAMPLE NO. 6A 7A 7B 8A 6.83 7.799.89 6.21 7.00 10.00 11.25 6.98 5.81 11.27 12.11 7.11 6.08 10.29 11.197.04 5.42 9.35 11.02 7.09 6.92 8.60 13.24 7.44 AVERAGE: 6.34 9.55 11.456.98 SAMPLE NO. 9 8.40 7.14 7.95 6.87 7.57 7.86 AVERAGE: 7.63

Having thus described the invention, what is claimed is:
 1. Anirradiated multiple layer polymeric film comprising: a. an outerprotective layer comprised of a blend of from at least 50% to 100%ethylene vinyl acetate resin with 6% to 12% vinyl acetate content,wherein the incipient cross-linking of said ethylene vinyl acetate resinof the outer protective layer occurs at about 1 to 2 MR and a gelfraction of at least 0.15 is present at 6 MR; b. an inner heat sealantlayer comprised of at least 50% to 100% of a material selected from thegroup consisting of ultra low density polyethylene, low densitypolyethylene, linear low density polyethylene and ionomers, wherein theincipient cross-linking of the material of said inner heat sealant layeroccurs at about 5 MR or higher; wherein said film is irradiated at aminimum radiation dose level of at least 1.5 megarads, wherein saidouter protective layer undergoes more cross-linking than said innersealant layer, with the proviso that no anti-oxidant cross-linkinginhibitor is present.
 2. A multiple layer film according to claim 1wherein said film is subjected to from about 2 MR to about 10 MR ofradiation.
 3. A multiple layer film according to claim 1 wherein saidfilm is oriented and subjected to from about 2 MR to about 10 MR ofradiation.
 4. A multiple layer film according to claim 1 wherein saidheat sealant layer is comprised of from about 80% to about 100% of amaterial selected from the group consisting of ultra low densitypolyethylene, low density polyethylene, linear low density polyethylene,very low density polyethylene and ionomers.
 5. A multiple layer filmaccording to claim 1 wherein said protective layer is comprised of blendof from about 80% to about 90% ethylene vinyl acetate and from about 10%to about 20% of a material selected from the group consisting of ultralow density polyethylene, low density polyethylene, linear low densitypolyethylene, very low density polyethylene and ionomers.
 6. A multiplelayer film according to claim 1 further comprising a barrier layerdisposed between said inner heat sealant layer and said outer protectivelayer.
 7. A multiple layer film according to claim 6 wherein saidbarrier layer is a polyvinylidene chloride copolymer.
 8. A multiplelayer film according to claim 7 wherein said polyvinylidene chloridecopolymer is a copolymer of polyvinylidene chloride and methylacrylate.9. A multiple layer film according to claim 6 wherein said barrier layeris ethylene vinyl alcohol.
 10. A heat shrinkable bag made in accordancewith claim
 1. 11. A heat shrinkable bag made in accordance with claim 7.12. A heat shrinkable bag made in accordance with claim
 8. 13. A heatshrinkable bag made in accordance with claim
 9. 14. An irradiatedmultiple layer film comprising: a. an outer protective layer comprisedof a blend of from at least 50% to 100% ethylene vinyl acetate resinwith 6% to 12% vinyl acetate content, wherein the incipientcross-linking of said ethylene vinyl acetate resin of the outerprotective layer occurs at about 1 to 2 MR and a gel fraction of atleast 0.15 is present at 6 MR; b. an inner heat sealant layer comprisedof at least 50% to 100% of a material selected from the group consistingof ultra low density polyethylene, low density polyethylene, linear lowdensity polyethylene and ionomers, wherein the incipient cross-linkingof the material of said inner heat sealant layer occurs at about 5 MR orhigher; c. a barrier layer disposed between said outer protective layerand said inner heat sealant layer; d. a first adhesive layer disposedbetween said inner heat sealant layer and said barrier layer; e. asecond adhesive layer disposed between said outer protective layer andsaid barrier layer; wherein said film is irradiated at a minimumradiation dose level of at least 1.5 megarads, wherein said outerprotective layer undergoes more cross-linking than said inner sealantlayer, with the proviso that no anti-oxidant cross-linking inhibitor ispresent.
 15. An irradiated multiple layer film according to claim 14wherein said first adhesive layer is comprised of a blend of from 50% to100% EVA resin and from 0% to 50% of a material selected from the groupconsisting of ULDPE, LDPE, LLDPE, VLDPE and ionomers.
 16. An irradiatedmultiple layer film according to claim 14 wherein said second adhesivelayer is comprised of at least from 50% to 100% of a material selectedfrom the group consisting of ULDPE, LDPE, LLDPE, VLDPE and ionomers. 17.An irradiated multiple layer film according to claim 14 wherein saidbarrier layer is comprised of a polyvinylidene chloride copolymer. 18.An irradiated multiple layer film according to claim 14 wherein saidbarrier layer is comprised of methylacrylate-polyvinylidene chloridecopolymer.
 19. An irradiated multiple layer film according to claim 14wherein said barrier layer is comprised of ethylene vinyl alcohol. 20.An irradiated multiple layer film according to claim 17 wherein saidfilm is subjected to from about 2 MR to about 10 MR of irradiation. 21.An irradiated multiple layer film according to claim 17 wherein saidfilm is oriented and subjected to from about 2 MR to about 10 MR ofirradiation.
 22. An irradiated multiple layer film according to claim 18wherein said film is subjected to from about 2 MR to about 10 MR ofirradiation.
 23. An irradiated multiple layer film according to claim 18wherein said film is oriented and subjected to from about 2 MR to about10 MR of irradiation.
 24. An irradiated multiple layer film according toclaim 19 wherein said film is subjected to from about 2 MR to about 13MR of irradiation.
 25. An irradiated multiple layer film according toclaim 19 wherein said film is oriented and subjected to from about 2 MRto about 10 MR of irradiation.
 26. A heat shrinkable bag made from anirradiated multiple layer film in claim
 20. 27. A heat shrinkable bagmade from an irradiated multiple layer film in claim
 21. 28. A heatshrinkable bag made from an irradiated multiple layer film in claim 22.29. A heat shrinkable bag made from an irradiated multiple layer film inclaim
 23. 30. A heat shrinkable bag made from an irradiated multiplelayer film in claim
 24. 31. A heat shrinkable bag made from anirradiated multiple layer film in claim
 25. 32. An irradiated multiplelayer polymeric film comprising: a. an outer protective layer comprisedof a blend of from at least 50% to 100% ethylene vinyl acetate resinwith 6% to 12% vinyl acetate content, wherein the incipientcross-linking of said ethylene vinyl acetate resin of the outerprotective layer occurs at about 1 to 2 MR and a gel fraction of atleast 0.15 is present at 6 MR; b. an inner heat sealant layer comprisedof at least 50% to 100% of a material selected from the group consistingof ultra low density polyethylene, low density polyethylene, linear lowdensity polyethylene and ionomers, wherein the incipient cross-linkingof the material of said inner heat sealant layer occurs at about 5 MR orhigher; said film is irradiated at a minimum radiation dose level of atleast 1.5 megarads, wherein said outer protective layer undergoes morecross-linking than said inner sealant layer, prepared by the methodcomprising: c. coextrusion of said outer protective layer and said innerheat sealant layer to form a film; d. orientation of said formed film byeither the blown tubular orientation process or the stretch orientationprocess to form an oriented film; and e. irradiation of said orientedfilm, with the proviso that no anti-oxidant cross-linking inhibitor ispresent.